No, changes can’t be made to an order after it’s placed. You must opt into the Samsung Care+ offer before proceeding to checkout on Samsung.com.
However, if you’d like to get the Samsung Care+ offer, please consider placing a new preorder on Samsung.com or the Shop Samsung app before pre-order ends (1). After selecting which device you like to purchase, please click +Add under Samsung Care+ and proceed to checkout. You can then cancel your previous order on Samsung.com.